Concerning the Reaction Pathway of the Metathesis Reaction

Oct 9, 2009 - The original “chop-chop” reaction reported by Schrock [J. Am. Chem. Soc. 1982, 104, 4291] involving W2(OtBu)6 and organic nitriles, ...
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10358

Inorg. Chem. 2009, 48, 10358–10363 DOI: 10.1021/ic901528c

Concerning the Reaction Pathway of the Metathesis Reaction involving WW and CN Triple Bonds: A Theoretical Study Shentan Chen and Malcolm H. Chisholm* Department of Chemistry, The Ohio State University Columbus, Ohio 43210 Received July 31, 2009

The original “chop-chop” reaction reported by Schrock [J. Am. Chem. Soc. 1982, 104, 4291] involving W2(OtBu)6 and organic nitriles, RCtN to give the metal alkylidyne and nitride products (tBuO)3WCtR and (tBuO)3WCtN, has been examined by a density functional theory based calculation where the bulky tBuO ligands have been substituted by MeO. The reaction between W2(OMe)6 and MeCN proceeds via a ditungstaazacyclobutadiene intermediate having a planar W2CN core, I, with a structure related to that seen for Mo2(OCH2tBu)6(μ-NCNMe2). Another possible intermediate having a pseudo tetrahedral W2CN core, II, a ditungstaazatetrahedrane was examined and shown to have a higher energy. The interconversion of I and II was found to be energetically unfavorable with respect to their formation of metathesis products. The highest energy transition state involving the conversion of I to products was comparable to that for the conversion of II to products but the initial formation of I from the reaction between W2(OMe)6 and MeCN was favored over the formation of II. The related reaction between Mo2(OMe)6 and MeCN was shown to be thermodynamically unfavorable with respect to either adduct formation or metathesis products. However the reaction between Mo2(OMe)6 and Me2NCN did yield a thermodynamically favored 1:1 adduct with a structure related to I.

Introduction 1

The original report by Schrock et al. of the metathesis reaction involving the WtW bond in W2(OtBu)6 with alkynes and nitriles, eq 1 and 2 below, led to the facile synthesis of discrete molecular species (tBuO)3WtCR that were active in alkyne metathesis, a reaction that has found considerable synthetic utility.2 W2 ðOt BuÞ6 þRCCR f 2ðt BuOÞ3 WCR W2 ðOt BuÞ6 þRCN f ðt BuOÞ3 WCRþðt BuOÞ3 WN

ð1Þ ð2Þ

Subsequently work in the Chisholm group established that in a number of instances there exists an equilibrium involving the tungsten alkylidyne and a dinuclear μ-alkyne adduct, a dimetallatetrahedrane.3 This equilibrium could be driven in favor of the alkylidyne by donor ligands and toward the alkyne adduct by the addition of a π-acceptor ligand such as CO. In a formal sense the interconversion of the alkylidyne *To whom correspondence should be addressed. E-mail: chisholm@ chemistry.ohio-state.edu. (1) Schrock, R. R.; Listemann, M. L.; Strugeoff, L. G. J. Am. Chem. Soc. 1982, 104, 4291. (2) (a) Schrock, R. R. Chem. Rev. 2002, 102, 145. (b) Schrock, R. R. Angew. Chem., Int. Ed. 2006, 45, 3748. (3) (a) Chisholm, M. H.; Folting, K.; Hoffman, D. M.; Huffman, J. C. J. Am. Chem. Soc. 1984, 106, 6794. (b) Chisholm, M. H.; Conroy, B. K.; Huffman, H. C.; Marchant, N. S. Angew. Chem., Int. Ed. Engl. 1986, 25, 446.

pubs.acs.org/IC

Published on Web 10/09/2009

and alkyne adducts could be viewed as an internal redox reaction. Following on from this work, Chisholm and Davidson examined the reaction pathway involving the interconversion of the dimetallatetrahedrane and the alkylidyne metal complexes in a computational study involving density functional theory on the model reaction shown in eq 3,4 W2 ðOHÞ6 ðμ-C2 H2 Þh2½ðOHÞ3 WCH

ð3Þ

More recently we have been interested in the chemistry of the metal nitrides of the group 6 elements, (RO)3MtN and their reactivity toward nitrogen atom metathesis with organic nitriles.5-7 This reaction, which proceeds more readily for M = W than for M = Mo8 and does not occur for M = Cr, was found to proceed by way of a diazametallacyclobutadiene intermediate, a nitrogen analogue of the metallacyclobutadiene that is involved in alkyne metathesis. These studies led us to question whether or not the metathesis reaction involving WtW and RCtN bonds, eq 2, followed a similar (4) Chisholm, M. H.; Davison, E. R.; Quinlan, K. B. J. Am. Chem. Soc. 2002, 124, 15351. (5) Chisholm, M. H.; Delbridge, E. E.; Kidwell, A. R.; Quinlan, K. R. Chem. Commun. 2003, 126. (6) Burroughs, B. A.; Bursten, B. E.; Chen, S.; Chisholm, M. H.; Kidwell, A. R. Inorg. Chem. 2008, 47, 5377. (7) Chen, S.; Chisholm, M. H.; Davidson, E. R.; English, J. B.; Lichtenberger, D. L. Inorg. Chem. 2009, 48, 828. (8) Gdula, R. L.; Johnson, M. J. A.; Ockwig, N. W. Inorg. Chem. 2005, 44, 9140.

r 2009 American Chemical Society

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Table 1. Selected Bond Distances (A˚) and Bond Angles (deg) for the Calculated Structure of W2(OMe)6(μ-NCMe) (I) and Those in the Solid State Structure of Mo2(OCH2tBu)6(μ-NCNMe2)9 A

B

dist (cal.)

dist (exp.)

A

B

C

angl (cal.)

angl (exp.)

M(1) M(1) M(1) M(1) M(1) M(1) M(2) M(2) M(2) M(2) M(2) N(3)

M(2) N(3) C(4) O(5) O(7) O(9) N(3) O(9) O(11) O(13) O(15) C(4)

2.554 1.970 1.948 1.913 1.913 2.324 2.081 2.001 1.959 1.907 1.907 1.367

2.449 1.908 2.014 1.908 1.918 2.146 2.134 1.999 1.954 1.872 1.870 1.333

N(3) M(1) M(1) M(1) M(2) M(2) M(1) M(1) O(9) O(9) M(1) M(2)

C(4) C(4) C(4) M(2) N(3) M(1) N(3) O(9) M(1) M(2) O(9) O(9)

C(17)a C(17)a N(3) N(3) C(4) C(4) M(2) M(2) N(3) N(3) C(10) C(10)

125.1 164.5 70.4 49.0 146.8 93.8 78.1 71.9 101.1 108.9 154.2 133.9

127.1 167.0 65.9 48.6 148.8 96.7 74.3 72.3 108.1 105.2 135.2 128.6

C(17) is a N atom in the solid state structure Mo2(OCH2tBu)6(μ-NCNMe2). a

Figure 1. Optimized structure for the ditungstaazacyclobutadiene (I). The hydrogen atoms are omitted for clarity.

path to that of the WtW þ RCtCR reaction in eq 1. We report here our findings that were prompted by this line of questioning. Results and Discussion 1:1 Adducts. We started by comparing the calculated ground state structures for the ditungstaazacyclobutadiene, I, and the ditungstaazatetrahedrane, II. A view of the optimized structure of I is given in Figure 1 where a striking similarity is seen with the molecular structure of Mo2(OCH2tBu)6(μ-NCNMe2)9 found in the solid-state. A comparison of related bond distances and angles pertaining to the central core is given in Table 1. Of singular note is the asymmetric or semibridging alkoxide with W-O distances of 2.001 and 2.324 A˚. The comparison of the metric parameters given in Table 1 is pertinent because the radii of molybdenum and tungsten are virtually identical due to the lanthanide contraction. The W-W distance of 2.554 A˚ in I is shorter than that in dimetallatetrahedrane W2(μ-C2H2)(OtBu)6(py)3 where W-W = 2.665(1) A˚. In the latter molecule this distance has been compared to that of a W-W single bond with the ethyne being reduced to C2H24-. In the case of I the C-N bond distance is lengthened from 1.171 A˚ in free MeCN to 1.367 A˚, essentially an increase of 0.2 A˚. This together with N-C-C angle of 125.1° is a clear indication of the reduction of the CtN bond and an oxidation of the (WW)6þ center. Finally, we note that the calculated free energy of I is 2.9 kcal/mol relative to the starting materials. However, the energies for the calculated structures for the dimethylcyanamide adducts M2(OMe)6(μ-NCNMe2) molecules were found to be stable with respect to the starting materials: ΔG° = -8.7 kcal/mol and -4.3 kcal/mol for M = W and Mo, respectively. The calculated structures for these two molecules are given in the Supporting Information. (9) Chisholm, M. H.; Huffman, J. C.; Marchant, N. S. Organometallics 1987, 6, 1073.

Figure 2. Optimized structure for the ditungstaazatetrahedrane (II). The hydrogen atoms are omitted for clarity.

The calculated structure for the ditungstaazatetrahedrane II is shown in Figure 2. The disposition of the six alkoxides around the W2 center is very similar to that seen in the molecular structure of W2 (OiPr)6(μ-C2H2)(py)23 where the loosely bound py molecules are trans to the bridging ethyne. The W-W distance in II is 2.572 A˚ which is similar to that in the ethyne adduct 2.567(1) A˚. A comparison of the metric parameters of the central cores of these molecules is given in Table 2. Again the lengthening of the C-N bond to 1.435 A˚ and the N-C-C = 126.1° is a clear indication of the reduction of the CtN by the W2 center. The calculated free energy of II is 10.0 kcal/mol, clearly indicating it is less stable than its isomer I. Pathway to Cleavage. Starting from I, which has a structure related to Mo2(OCH2tBu)6(μ-NCNMe2),9 we have traced the path to cleavage yielding the isolated species (MeO)3WtN and (MeO)3WtCMe by the free energy profile shown in Figure 3. What becomes immediately obvious is that there are two intermediates that have a significantly lower energy than that of I and furthermore that one of them is lower in energy than

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Table 2. Selected Bond Distances (A˚) and Bond Angles (deg) for the Calculated Structure of W2(OMe)6(μ-NCMe) (II) and Those in the Solid State Structure of W2(OiPr)6(μ-C2H2)(py)23 A

B

dist (cal.)

dist (exp)

W(1) W(1) W(1) W(1) W(1) W(1) W(1) W(2) W(2) W(2) W(2) W(2) W(2) C(4)

W(2) O(5) O(7) O(9) O(11) N(3)a C(4) N(3)a C(4) O(9) O(11) O(13) O(15) N(3)a

2.572 1.898 1.907 2.188 2.110 2.073 2.061 2.073 2.061 2.189 2.110 1.898 1.907 1.435

2.567 1.948 1.927 2.114 2.175 2.080 2.096 2.094 2.118 2.099 2.183 1.950 1.940 1.394

a

A

B

C

angl (cal.)

angl (exp)

N(3)a W(1) W(1) W(2) W(2) W(1) W(1) W(1) C(4) C(4) C(4) W(2) C(4) O(5) O(13)

C(4) C(4) W(2) N(3)a W(1) C(4) N(3)a O(9) W(2) W(1) W(2) O(11) W(1) W(1) W(2)

C(17) N(3)a N(3)a C(4) C(4) W(2) W(2) W(2) O(9) O(9) O(11) W(1) O(11) O(7) O(15)

126.1 70.1 51.7 69.3 51.4 77.2 76.7 72.0 101.2 101.2 86.1 75.1 86.1 95.9 95.9

; 69.9 51.8 71.6 52.9 75.1 75.9 75.1 104.2 104.5 77.6 72.2 78.2 92.2 92.5

N(3) is a C atom in the solid structure W2(OiPr)6(μ-C2H2)(py)2.

Figure 3. Free energy (calculated at 298 K in kcal/mol) profiles for the cleavage of ditungstaazacyclobutadiene (I). P denotes the isolated species (MeO)3WtN and (MeO)3WtCMe.

the products, namely, P6. An examination of P6 shows that it is simply an alkoxide bridged molecule in which the WtN and WtCMe bonds are already formed. There is an additional weak W 3 3 3 N interaction of 2.134 A˚. It is not unreasonable that this should be lower in energy since, in the reaction between MeCtN and W2(OtBu)6, the nitride product (tBuO)3WtN is an infinite linear polymer involving alternating short and long W-N distances.10 Also in the solid state the structure of (tBuO)3WtCMe is a dimer with a pair of alkoxide bridges that have alternating long and short W-O bonds with the long bonds being trans to the WtCMe moiety.10 The structure of the other lower energy intermediate, P4, is shown in Figure 4 and is closely related to that of I, although the W-W distance is longer 2.653 A˚, the W-C distance 1.929 A˚ is slightly shorter, and the W-N distances move in the direction of the cleavage, namely, one gets longer and the other shorter. The C-N bond distance is relatively unperturbed, but the bridging methoxide is (10) Chisholm, M. H.; Hoffman, D. M.; Huffman, J. C. Inorg. Chem. 1983, 22, 2903.

Figure 4. Optimized structure for P4. The hydrogen atoms are omitted for clarity.

more symmetrically bridging the two W atoms [W(1)O(9) = 2.153 A˚, W(2)-O(9) = 2.152 A˚] and is more sp3 hybridized such that the methyl carbon does not lie in the W2O plane. The highest energy transition state, P5 can be seen to be derived from P4 by the rupture of the C-N bond, C 3 3 3 N = 2.016 A˚, with an accompanying significant lengthening of the longer W-N distance, 2.158 to 2.293 A˚, and a shortening of the other, 1.921 to 1.788 A˚. The W-C distance similarly shortens form 1.929 to 1.800 A˚ and the W-C-C angle is enlarged from 153.7 to 177.2°. The W-W distance also increases to 2.653 to 2.782 A˚, which is approaching a non-bonding distance with the two tungsten atoms being bridged by the single alkoxide. In the other transition states, the motion involves primarily W-O-C bending and rotation. Selected structural data for this reaction pathway are given in Table 3. The reaction pathway for II leading to cleavage is more complex and is shown in Figure 5. There are many relatively inconsequential energy wells and saddle points that merely involve perturbation of W-O-C groups but the two key high energy transition states are represented by V1 and V9. The change from II to V1 represents a twisting of the μ-NCMe ligand from the perpendicular toward the parallel mode of bonding as is often seen for non-parallel, non-perpendicular alkyne bridges.11 The structures of V2 and V8 are similar though V8 is lower in energy and in both there are still two bridging alkoxides. In V8 the central W2CN moiety is not rigorously planar. The two W-C and W-N distances are distinctly asymmetric, and the C-N distance of 1.417 A˚ is effectively a single bond distance. Selected structural data for the intermediates and transition states for the reaction path shown in Figure 5 are given in Table 4. Of note here is that the energy of V8 is 5.4 kcal/mol higher than that of I. The reaction from V8 to V10 represents the cleavage of the C-N bond and proceeds via the transition state V9, and occurs with a twisting of the two halves of the molecule. The structure of the (11) (a) Chisholm, M. H.; Click, D. R.; Gallucci, J. C.; Hadad, C. M.; Wilson, P. J. J. Am. Chem. Soc. 2002, 124, 14518. (b) Hoffmann, D. M.; Hoffmann, R.; Fisel, C. R. J. Am. Chem. Soc. 1982, 104, 3858. (c) Calhorda, M. J.; Hoffmann, R. Organometallics 1986, 5, 2181. (d) Cotton, F. A.; Feng, X. Inorg. Chem. 1990, 29, 3187. (e) Chisholm, M. H.; Lynn, M. A. J. Organomet. Chem. 1998, 550, 141.

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Table 3. Selected Bond Distances (A˚) and Bond Angles (deg) for Intermediates and Transition States on the Pathway Towards the Cleavage of Ditungstaazacyclobutadiene (I)

I P1 P2 P3 P4 P5 P6 P7 P8

W(1)-W(2)

C(4)-N(3)

W(1)-C(4)

W(1)-N(3)

W(2)-N(3)

W(1)-O(9)

W(2)-O(9)

C(17)-C(4)-N(3)

C(17)-C(4)-W(1)

2.554 2.662 2.602 2.626 2.653 2.782 3.265 3.671 2.999

1.367 1.365 1.331 1.345 1.364 2.016 3.039 3.002 3.903

1.948 1.922 1.951 1.939 1.929 1.800 1.768 1.775 1.772

1.970 1.998 2.032 2.043 2.158 2.293 2.134 2.206 2.206

2.081 2.020 2.041 1.996 1.921 1.788 1.741 1.710 1.707

2.324 2.804 3.285 2.826 2.153 2.112 2.176 2.024 1.957

2.001 1.967 1.962 2.009 2.152 2.205 2.150 3.089 4.070

125.1 127.5 129.5 128.4 126.4 107.2 136.5 133.8 135.4

164.5 159.5 156.7 157.0 153.7 177.2 179.0 179.4 177.6

Figure 5. Free energy (calculated at 298 K in kcal/mol) profiles for the cleavage of ditungstaazatetrahedrane (II). P denotes the isolated species (MeO)3WtN and (MeO)3WtCMe.

product of this C-N cleavage and twisting, V10, is interesting since, though it clearly has nitride and alkylidyne ligands on separate tungsten atoms, it is in other ways highly asymmetric. The molecule is shown in Figure 6, and structural data are given in Table 5. The asymmetry in the molecule is suggestive of a [(MeO)2WtN]þ cation associating with a [(MeO)4WtCMe]- anion where the alkylidyne and two of the attendant OMe ligands are semibridging to the positively charged tungsten center of the [(MeO)2WtN]þ cation. The reaction pathway leading from V10 to V18 involves further twisting and W-O-C maneuvering until the pseudo fused trigonal bipyramidal structure is attained wherein the WtN and WtCMe groups are mutually anti and occupy axial positions of a distorted trigonal bipyramid. This structure is analogous to that seen for the (tBuO)3WtCMe dimer.9 As in the reaction for II, the last step in the reaction, the cleavage of the alkoxide bridged dimer, is endothermic. In comparing the energetics of the two reaction pathways leading to cleavage we find that the highest energy transition states are comparable in energy. See P5 in Figure 3 and V1 in Figure 5. This is clearly not discriminating with respect to the pathway leading to C-N bond cleavage. We next looked at the reaction pathway from the free reactants toward the formation of P4 and V8. Pathway to Adduct Formation. The reaction pathway from the free reactants to the lowest energy 1:1 adducts is shown in Figure 7. The formation of the perpendicular and parallel modes of bonding follows a common reaction path leading to F5. The local minimum F1 corresponds to coordination of MeCN to one W atom via a weak W 3 3 3 N interaction. The transition state F2 corresponds to the repositioning of the nitrile toward bridge

formation. F3, F5, and P4 all contain the planar ditungstaazacyclobutadiene geometry and only differ with respect to the conformations of the W-O-Me groups. The reaction leading from F5 to the μ-perpendicular intermediate II proceeds via the dotted pathway in Figure 7. From this it can be seen that the minimum, V8, which has the non-perpendicular/non-parallel bridge, is accessed before the intermediate II. However, what becomes obvious is that the transition state T10 is higher in energy than the transition state for the cleavage of the C-N via the μ-parallel intermediate. The energy of P5 is 18.7 kcal/mol relative to the reactants as shown in Figure 3. From this we can conclude that the μ-parallel intermediates are involved in the reaction leading to CtN cleavage and not the μ-perpendicular ones. Mo2(OMe)6 þ MeCtN. We have also considered the reaction between the molybdenum triple bonded complex Mo2(OMe)6 and MeCtN. The reaction products (MeO)3MotN and (MeO)3MotCMe are higher in energy by 5.0 kcal/mol which is consistent with the lack of multiple bond metathesis involving MotMo and CtN bonds. Moreover, if we consider a similar reaction pathway for molybdenum then the transition state involving the C-N bond rupture (P5 in Figure 3) is calculated to be 33.3 kcal/mol which is 15 kcal/mol higher relative to when M = W. Mo2(OMe)6 þ Me2NCtN. For M = Mo, the formation of the 1:1 adduct of structure type I is calculated to be favorable by 4.3 kcal/mol relative to the starting materials and also favorable with respect to the cleavage products. For M = W, both the μ-parallel adduct of type I and the cleavage products are stable with respect to the reactants; ΔG° = -8.7 and -7.8 kcal/mol, respectively. For tungsten, with the bulky tBuO ligands, only the cleavage products are observed. This may well reflect steric factors destabilizing a structure akin to I and/or that the products of the reaction are not monomeric but rather associate by either MtNfM bonding or alkoxide bridge formation. Both factors clearly will favor the metathesis reaction over 1:1 adduct formation. W2X6 þ MeCN, where X = Cl and F. A clear complication in calculations employing alkoxide ligands is that they are variable π-donors. With an sp2-hybridized oxygen, the RO- ligand can be viewed as a σ2π2-single-faced π-donor whereas in the linear M-O-R group, the RO- ligand is a cylindrical σ2π4 donor.12 These two are clearly limiting descriptions and it is the variation and orientations of the (12) (a) Chisholm, M. H. Chemtracts-Inorganic Chemistry 4 1992, 273. (b) Eisenstein, O.; Hoffmann, R.; Rossi, A. R. J. Am. Chem. Soc. 1981, 103, 5582. (c) Wilker, C. N.; Hoffmann, R.; Eisenstein, O. Nouveau J. Chim. 1983, 7, 535.

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Table 4. Selected Bond Distances (A˚) and Bond Angles (deg) for Intermediates and Transition States on the Pathway Towards the Cleavage of Ditungstaazatetrahedrane (II)

II V1 V2 V3 V4 V5 V6 V7 V8 V9 V10 V11 V12 V13 V14 V15 V16 V17 V18

W(1)W(2)

C(4)N(3)

W(1)C(4)

W(2)C(4)

W(1)N(3)

W(2)N(3)

W(1)O(9)

W(2)O(9)

W(1)O(11)

W(2)O(11)

C(17)C(4)-N(3)

C(17)C(4)-W(1)

2.572 2.539 2.601 2.612 2.618 2.641 2.633 2.649 2.628 2.658 2.949 2.940 2.931 2.980 3.050 3.345 3.564 3.645 3.635

1.435 1.445 1.418 1.416 1.411 1.422 1.413 1.418 1.417 1.802 2.953 2.937 2.950 3.130 3.217 4.241 5.444 6.339 6.697

2.061 1.990 1.997 2.002 2.010 2.005 2.012 2.004 2.000 1.919 1.808 1.809 1.802 1.795 1.782 1.769 1.766 1.767 1.779

2.061 2.176 2.293 2.291 2.291 2.317 2.323 2.362 2.346 2.254 2.481 2.476 2.561 2.664 2.905 3.600 4.298 4.906 5.156

2.073 2.449 2.895 2.929 2.947 2.957 2.949 2.946 2.925 3.288 4.020 4.016 3.966 4.011 3.965 4.424 4.873 5.017 5.044

2.073 1.913 1.843 1.837 1.834 1.815 1.822 1.816 1.822 1.750 1.696 1.697 1.700 1.699 1.701 1.702 1.702 1.701 1.698

2.188 2.098 2.101 2.068 2.048 2.065 2.057 2.080 2.060 2.127 2.141 2.135 2.115 2.114 2.193 2.231 2.278 2.437 2.451

2.188 2.171 2.107 2.138 2.153 2.160 2.162 2.166 2.168 2.154 2.159 2.167 2.142 2.068 2.068 2.028 1.992 1.973 1.945

2.110 2.211 2.160 2.162 2.160 2.139 2.146 2.106 2.164 2.035 1.994 1.999 2.043 2.042 2.055 2.061 2.067 2.009 1.956

2.110 2.152 2.126 2.129 2.118 2.108 2.110 2.079 2.098 2.335 2.564 2.535 2.507 2.543 2.385 2.345 2.365 2.406 2.502

126.1 123.5 117.0 117.2 117.2 116.3 117.2 116.6 116.5 100.3 83.6 83.4 82.4 84.2 83.5 95.0 117.5 142.7 161.0

140.3 140.3 127.4 125.8 124.9 125.4 125.1 125.6 126.8 134.8 159.6 169.7 164.7 168.7 174.8 178.1 177.8 177.9 179.7

Figure 6. Optimized structure for V10. The hydrogen atoms are omitted for clarity. Table 5. Selected Bond Distances (A˚) and Bond Angles (deg) for the Calculated Structure of V10 A

B

W(1) W(1) W(1) W(1) W(1) W(1) W(1) W(2) W(2) W(2) W(2) W(2) W(2) C(4)

W(2) O(5) O(7) O(9) O(11) N(3) C(4) N(3) C(4) O(9) O(11) O(13) O(15) N(3)

dist(cal.) 2.949 1.909 1.894 2.141 1.994 4.020 1.808 1.696 2.481 2.159 2.564 1.908 1.899 2.953

A

B

C

W(1) W(1) W(1) W(2) W(2) W(1) C(4) C(4) C(4) W(2) C(4) O(5) O(13)

C(4) C(4) W(2) N(3) W(1) O(9) W(2) W(1) W(2) O(11) W(1) W(1) W(2)

C(17) N(3) N(3) C(4) C(4) W(2) O(9) O(9) O(11) W(1) O(11) O(7) O(15)

angl(cal.) 159.6 113.0 117.3 57.1 57.0 86.6 72.5 88.0 71.6 79.5 101.8 94.8 103.3

M-O-C angles that complicate the calculations presented above and, as seen in Figure 5 and 7, produce many local minima and transition states that are not “rate-limiting” in

Figure 7. Free energy (calculated at 298 K in kcal/mol) profiles leading to the formation of ditungstaazacyclobutadiene (I) and ditungstaazatetrahedrane (II) from the starting materials. The red solid line is the pathway leading to the formation of I; the blue dash line is the pathway leading to the formation of II.

the overall reaction pathway. Consequently, we wondered if the key features of the reaction could be reproduced by replacing the methoxide by the halides, Cl- and F-, even though the W2X6 compounds (X = Cl, F) are not known as triply bonded molecules with D3d symmetry. Starting with the optimized geometry for the X3WtWX3 molecules, we probed the reaction with MeCN, and this is presented in detail in the Supporting Information. The cleavage products X3WtCMe and X3WtN are not thermodynamically favored in either case: ΔG° = 4.6 and 6.9 kcal/mol for X = Cl and F, respectively. However, the transition state involving the C-N bond rupture was structurally similar to that for X = OMe and in both cases was the highest energy. In both cases the μ-parallel MeCN adduct akin to the ditungstaazacyclobutadiene was more stable than the μ-perpendicular adduct, the ditungstaazatetrahedrane, but the most stable 1:1 adduct (ΔG° = -21 kcal/mol) had the structure shown below (X = F). In this structure one W atom is in a pseudo square pyramidal geometry and the fluoride ligands are terminal.

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Me2ButSiOH with the loss of pyridine to give W2(μ-CCH)(OSiMe2But)5 with the conversion of the W2C2 moiety from perpendicular to parallel, μ-η1,η2.13 However, despite this difference in preferential bonding, the lowest energy transition state for both C-C and C-N cleavage are rather similar with an asymmetric structure in which there is one terminal W-alkylidyne and a bridging alkylidyne or nitride ligand. Computational Details

Given the high electronegativity of the fluoride ligands and the strong reducing power of the WtW bond, it is not surprising that the 1:1 adduct of W2F6 with MeCN is more stable that its methoxide counterpart. Concluding Remarks It is interesting now to compare the reaction pathways involving the metathesis of CtC and CtN bonds in their reactions with MtM triple bonds in M2(OR)6 compounds. The reaction favors the cleavage products only when M = W. This is probably a simple reflection of the greater stability of the M(6þ) oxidation state for the third row transition metal and the strength of metal-ligand bonding: 5d > 4d > 3d. In reactions involving alkynes, the μ-perpendicular 1:1 adduct, the dimetallatetrahedrane is more stable than the μ-dimetallacyclobutadiene but for nitriles the μ-parallel adducts, the dimetallaazacyclobutadiene intermediates, are thermodynamically favored over the μ-perpendicular, the dimetallaazatetrahedrane. It is in interesting to speculate why these 1:1 adducts are favored with differing structures. In our view the stability of the μ-parallel nitrile bonding bonding mode is favored for RCN because this maximizes metal-nitrogen bonding and in some ways this is similar to an acetylide which in a bridge bonding mode is often seen as μ-η1,η2. A nice example of this transformation is seen in the reaction of W2(μ-C2H2)(OSiMe2But)6(py) which eliminates (13) Chisholm, M. H.; Cook, C. M.; Huffman, J. C.; Streib, W. E. J. Chem. Soc., Dalton Trans. 1991, 929. (14) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A.; Vreven, T., Jr.; Kudin, K. N.; Burant J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Fresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G. Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challocombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, Revision C.02; Gaussian, Inc.: Wallingford, CT, 2004.

All calculations were performed using DFT as implemented in the Gaussian 03 suite of programs.14 The PW91PW9115-19 [Perdew and Wang’s 1991 exchange and gradient-corrected correlation functional] density functional were used for all calculations. The LanL2DZ20-22 was used for transition metal atoms, and the 6-31G* basis sets23-27 were used for all other non-metal atoms in the model compounds. All the structures were fully optimized without symmetry constraints. Frequency calculations were also performed to confirm that all the stationary points were minima or transition states (no imaginary frequency for a minimum and one imaginary frequency for a transition state). Intrinsic reaction coordinates (IRC)28,29 calculations were carried out on transition states to confirm these structures are indeed connecting two minima. The discussed energies are relative Gibbs free energies (ΔG298K). All the relative energies were defined with respect to the starting materials.

Acknowledgment. We thank the National Science Foundation and the Ohio State University for financial support and the Ohio Supercomputer Center for computational resources. S.C. also thanks Dr. Bruce E. Bursten for his input toward the Ph.D. Supporting Information Available: Calculated structures of minima and transition states associated with the reaction of W2(OMe3)6 and MeCN. Calculated structures of the 1:1 dimethylcyanamide tungsten adduct W2(OMe)6(μ-NCNMe2) and the molybdenum adduct Mo2(OMe)6(μ-NCNMe2). Energy profiles and the structures of the minima and transition states along the reaction pathways for the metathesis reactions between W2X6 (X = Cl, F) and MeCN. This material is available free of charge via the Internet at http://pubs.acs.org. (15) Burke, K.; Perdew, J. P.; Wang, Y. In Electronic Density Functional Theory: Recent Progress and New Directions; Dobson, J. F., Vignale, G., Das, M. P., Eds.; Plenum: New York, 1998; pp 81-111. (16) Perdew, J. P. In Electronic Structure of Solids ’91; Ziesche, P., Eschrig, H., Eds.; Akademie Verlag: Berlin, 1991; p 11. (17) Perdew, J. P.; Chevary, J. A.; Vosko, S. H.; Jackson, K. A.; Pederson, M. R.; Sing, D. J.; Fiolhais, C. Phys. Rev. B: Condens. Matter 1992, 46, 6671. (18) Perdew, J. P.; Chevary, J. A.; Vosko, S. H.; Jackson, K. A.; Pederson, M. R.; Singh, D. J.; Fiolhais, C. Phys. Rev. B: Condens. Matter 1993, 48, 4978. (19) Perdew, J. P.; Burke, K.; Wang, Y. Phys. Rev. B: Condens. Matter 1996, 54, 1653. (20) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270. (21) Wadt, W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284. (22) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299. (23) Ditchfield, R.; Hehre, W. J.; Pople, J. A. J. Chem. Phys. 1971, 54, 724. (24) Hehre, W. J.; Ditchfield, R.; Pople, J. A. J. Chem. Phys. 1972, 56, 2257. (25) Hariharan, P. C.; Pople, J. A. Mol. Phys. 1974, 27, 209. (26) Gordon, M. S. Chem. Phys. Lett. 1980, 76, 163. (27) Hariharan, P. C.; Pople, J. A. Theor. Chim. Acta 1973, 28, 213. (28) Fukui, K. J. Phys. Chem. 1970, 74, 4161. (29) Fukui, K. Acc. Chem. Res. 1981, 14, 363.